How fresh was the extract? Liquid extract has a distinct lifespan, and the hopped type degrades even more markedly. (If almost all hop aroma degrades durring the course of a 1 hour boil, leaving only the bitterness, imagine what happens to the hop profile durring the dehydration process!) I once made a prehopped mr. beer Old english bitter, and it had a distinct cidery, sour taste. I am quite sure that it wasn't a infection, just stale ingredients. For better taste, use unhopped malt extract, from a source that is sure to get you fresh stuff, add your own fresh hopps, and steep some crushed specialty grains to lend the wort a fresher flavor. It'll be twice as good as the prehopped junk. There are lots of folks on here doing the partial mash/ grain steeping method to help you out there. Good luck on your future brews!